A notebook Mac uses up to the entire output of the power adapter. If more is needed for intensive computation and processing, it freely "borrows" power from the battery. It does this on a moment by moment basis, and it generally is does not show you that this is happening. When you ask that particular high-powered notebook Computer to do graphics intensive operations, It may appear that the charge level on the battery is going down. That is because it IS going down.
Your computer is working as designed. If it did not "borrow" from the battery, the power adapter would have to be MUCH larger, or your ability to do computations would be severely throttled.
If you then proceed to use a power adapter (the one in the display) as the ONLY power adapter, and it provides LESS that the required capacity for the computer, unexpected things can happen. Apple recommends you use a power adapter rated at least as high as the original power adapter specified for the machine. Nothing else is considered acceptable.
In some cases, using a too-small power adapter will appear to work, but will draw down battery power very rapidly.
In extreme cases, your Mac could over-draw the available power. Over-drawing the available power will result in an emergency, uncontrolled power-down, as your Mac takes steps to save itself from damage.
The recommendation has always been, and continues to be, to connect a power adapter of at least the minimum specified for your computer. Apple does not provide any guidance for connecting multiple adapters, but Users report it works.
Find the right power adapter and cable for your Mac notebook - Apple Support
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